Image frequency suppression for superheterodyne receivers



Patented Qctflfi, 1934 reins IMAGE FREQUENCY SUPPRESSIQN SUPERHETERQDYNE RECEIVERS Garrard Mountjoy, Jackson, Mich, assignor, by I mesh-e assignments, to Radio Corporation of diner-lea, New York, N. 51., a corporation of Delaware Application March 31, 1933, Serial No. 663,700

' '2' Claims. (Cl. 250-420) This inventionrelates to superheterodyne receiving sets in which the oscillator produces an ity.

In the frequency scale the oscillator frequency, lies midway between the desired signal frequency and an undesired signal frequency called the image frequency.

It is an object of i this invention to lead the signal and image frequencies through one path consisting of a tuned circuit and a vacuum tube amplifier and then introduce through another path an opposing image frequency to reduce or eliminate the image frequency coming through 15 the amplifier path.

The manner in which the result is accomplished is illustrated in the drawing in which the figure shows a superheterodyne circuit embodying th invention.

Referring to the drawing in detail:

The antenna 1 adapted to receive the signal modulated carrier frequency is connected through coupling condenser 2 to the tank circuit or tuned loop consisting of inductance 3 and variable condenser 4. One side of the loop is grounded and connected to the cathode 5 of amplifier tube 6.

The grid '7 of this is connected through a negative biasing source 8 to the other end of the tank circuit. The plate or anode 9 is connected through a transformer primary 10 and source of voltage 11 to the cathode 5. The secondary 12 of the transformer is tuned by condenser 13 to the signal modulated carrier frequency and is connected at one side to the cathode 14 and at the other to the grid 15 through the negative biasing voltage source 16. The plate 17 is connected through primary coil 18 and sourceof voltage 19 to the cathode 14. The secondary 20 of the transformer is tuned to the signalImodulated carrier frequency by condenser 21. This tank circuit is connected to the first detector 22 where the oscillator 23 reacts with the signal modulated carrier frequency, and the image were it not for my improvement, to produce the intermediate frequency. This intermediate frequency is amplified by one or more amplifiers 24 tuned at the factory to this frequency and the output is fed into the second or audio detector 25. The output of the detector is amplified by one or more audioamplifiers 26 and converted into sound by a loud speaker or other device 27.

The circuit connections following the tuned loop 20, 21, are not shown with circuit details as those details are unnecessary foran understanding of the invention.

intermediate frequency, or beat, of superaudioil- The arrangement thus far described concerns the old and well known superheterodyne circuit and my invention consists in a bucking circuit connected between the antenna and a point subsequent to one or more amplifying tubes. Preferably the circuit is connected to the plate of the first amplifier tube and consists of a resistance 28 having a tap 29 connected to the plate 9. A branch circuit contains an inductance 3B and condenser 31 connected to the resistance 28 by tap 32. The inductance 30 and condenser 31 are adjustable. The taps 29 and 32 also enable the resistance portions 33 and 34 to be adjusted. By sliding tap 32 to the left hand terminal of the resistance the antenna-to-plate path consists only of resistance and the amount of this resistance will be determined by'the position of the tap 29. In many cases I have secured a sufiicient image reduction by use of resistance alone and when the proper amount of resistance has been found all other sets similarly constructed may have a fixed resistance of this same value. In such sets an adjustable resistance will ordinarily be unnecessary.

As a still greater refinement for more complete 30. elimination of the image, the tap 32 may be adjusted to the right in the figure so that the impedance of the branch contains not only resistance 34 but also the inductance S0 and capacity 31 shunted by resistance 33 or not, as a trial will 35; determine. By opening the switch 35 this shunt resistance may be eliminated. The inductance, resistance and capacity are varied on i the image frequency is minimized to the greatest extent and then these values of inductance, resistance and capacity may be used in identical sets without making them adjustable.

By use of vector equations or other form of mathematics the values of the resistances, inductance and capacity alone or in any desired combination may be approximately determined but the mathematical proof is complex and I find it preferable to determine the values empirically as described.

When voltages of signal and image frequency V are induced in the antenna those voltages reach the point 36 through two paths. The potentials produced through the tank circuit 3, 4, and amplifying tube 6 have the voltage of the signal much increased in respect to that of the image because the tank circuit is tuned to the signal. The image frequency therefore does not resonate.

In the parallel path, with either resistance alone or with capacity and inductance as determined by test for the particular circuit involved,

nal andimage and by adjustment of theiinpedance asdescribed, the phase and value of the image arriving at the point 36 through the last mentioned path is equal and opposite to the value of the image arriving through thetank circuit and amplifying tube. The image therefore-"disappears. Since the .signal has been resonated and amplified in the amplifier circuit the signal in this circuit greatly overpowers the signal com-' ing through thebucking circuit' and thus is not eliminated, even should the two" be completely out of phase. The circuits following the point 36 therefore contain only the signal or such a predominating amount of the signal thatthe image is no longer a troublesome factor. r In the drawing the grid, filament and plate sources are shown as batteries but this is for siinplification only. In most cases power supply apparatus would furnish rectified voltages from the house supply mains. 1, Such power supply apparatus would alsosupply the'grid filament and plate circuits of the tubes and speaker not shown in complete detail.

Having described my invention what; mm,

1, In superheterodyne receiving sets, an an-- there will be. no discrimination between the sig-' tenna, an oscillator adapted to beat with the signal and image frequencies, circuit connections between said antenna and said oscillator containing two parallel paths over which the signal and image frequencies tend to pass to combine with image frequencies tend to pass to combine with the oscillator frequency, one of said paths containingatuned circuit and a vacuum tube amplifier and the other path containing resistance andreactance to produce at the output junction points of said paths an image voltage equal and opposite to the imagevoltage produced by the first mentioned path. I

GARRARD MOUNTJOY. 

